Method for making aluminum sheet

ABSTRACT

A method for forming aluminum sheet material containing 0.5-3.8 percent magnesium, having little or no propensity for forming ripples when subsequently plastically stretched during fabrication said method comprising hot rolling, intermediate annealing to effect substantially complete recrystallization and subsequently cold rolling to final gauge thickness and the product formed thereby.

D United States Patent 51 3,661,657 Wong 51 May 9, 1972 [54] METHOD FORMAKING ALUMINUM 3,359,085 12/1967 Anderson ..148/11.5 A SHEET 3,502,4483/1970 Anderson et al... ..148/32 3,560,269 2/1971 Anderson et al 148/11.5 A [72] Inventor: Winston A. Wong, Livermore, Calif. [73] Assignee:Kaiser Aluminum & Chemical Corpora- 'f 'f Rutledge on, Oakland Calif:Assistant E.\ammerW. W. Stallard Attorney-James E. Toomey, Paul E.Calrow, Harold L. Jen- [22] Filed: Dec. 7, 1970 kins and Edward J. Lynch[21] App]. No.: 95,930 ABSTRACT [52] U 5 Cl 148/32 145ml 5A A method forforming aluminum sheet material containing [51] 1/04C22c zi/oo 0.5-3.8percent magnesium, having little or no propensity for 58 1 Field 1 48/1A 159 32 forming ripples when subsequently plastically stretched duringfabrication said method comprising hot rolling, intermediate 56]References Cited annealing to effect substantially completerecrystallization UNITED STATES PATENTS 2/1966 Anderson 148/1 1.5 A

and subsequently cold rolling to final gauge thickness and the productformed thereby.

3 Claims, No Drawings BACKGROUND OF THE INVENTION This invention relatesto a process for producing aluminum sheet material free of rippledsurface markings and the like when the sheet material is plasticallystretched during subsequent fabrication procedures. Fabricationprocedures which employ stretching as a prime mode of deformationinclude stretch forming and drawing operations. These types offabricating procedures are frequently used for aluminum materials in theproduction of automotive products such as wheel openings, door andwindow trim, hub caps and the like. ln these procedures, the aluminummaterial is frequently heated to a temperature between about 400 and 550F. to increase the formability of the aluminum materials.

The surface finish of aluminum sheet material is particularly importantin automotive applications. However, it has been found that aluminumsheet material exhibits a rippled surface after stretch forming and thelike which detract from its appearance. This is particularly noticeableafter the shapes are anodized. The ripples are defined herein as ridgesextending as parallel surface markings in a direction transverse to therolling direction of the aluminum sheet material. Apparently, the rippleformation occurs only when the principal direction of deformation duringfabrication is the same as the rolling direction.

,In the past, automotive trim stock has been prepared by hot rolling thealuminum material to an intermediate gauge thickness (e.g. 0.1 in.,)cooling to room temperature and cold rolling to the desired finalthickness, normally about 0.03 in. Prior to hot rolling the material isgiven the normal treatments such as homogenizing, scalping, and aninitial breakdown. When the automotive trim stock is stretch fabricatedinto the desired shape, it has been necessary to buff the shapes with afine abrasive material to remove the rippled appearance. Usually theshapes are complex which make it difficult to buff adequately to removethe ripples. Most, if not all, automotive trim applications for aluminumand aluminum alloys require that the automotive trim be bright dipped ina solution of hydrofluoric acid and nitric acid and then clear anodizedin a 15 percent sulfuric acid electrolyte to form an oxide coatinghaving a nominal thickness of 0.3 mil. The finishing steps furtheraccentuate the rippled appearance of the surface.

SUMMARY OF THE INVENTION The present invention relates to a method formaking aluminum sheet material which does not have the propensity toform ripples when subsequently stretched during fabricating procedures.The alloy composition of the sheet material consists essentially ofabout 0.5-3.8 percent magnesium and the balance aluminum, incidentalelements and impurities. Preferably the alloy contains from about 2.03.6percent magnesium.

In the process of the present invention, after the alloy is subjected tothe usual pretreatment such as homogenizing, scalping and initialbreakdown, the material is hot rolled, intermediate annealed to effectsubstantially complete recrystallization and subsequently cold rolled toefi'ect a reduction of thickness between about 50 and 90 percent of thehot rolled thickness.

The sheet material formed by the process of the present in vention ischaracterized by tensile strength between about 29,000 and 53,000 psi,an elongation of between I and percent and further characterized by theabsence of transgranular microscopic shear bands at or near the surfacethereof which occur transverse to the rolling direction at an anglebetween about 30 and 45 from the surface when viewing a longitudinalcross-section of the sheet material. By X-ray analysis it has been foundthat the microscopic structure of the product of this invention containsbetween about 5 and 30 percent cube texture component, the remainingbeing the normal rolling texture.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directedto a process for making a thin sheet of an aluminum-magnesium alloyconsisting essentially of from about 0.5 to 3.8 percent magnesium andthe balance aluminum, incidental elements and impurities. Preferably themagnesium level is maintained between 2.0 and 3.6 percent. Theimpurities and incidental elements should not exceed the limits of up to0.10 percent silicon, up to 0.15 percent iron, up to 0.10 percentcopper, up to 0.30 percent manganese, and other elements up to 0.05percent each, up to 0.15 percent total.

In accordance with this invention, after the usual pretreatment such ashomogenizing, scalping and initial breakdown, the above-described alloyis hot rolled at an initial temperature between 800 1,000 P. to anintermediate gauge thickness. The intermediate gauge thickness is thenannealed to effect complete recrystallization by heating to atemperature between about 575 and 950 F. for a period of time betweenabout 2 and 24 hours. Preferably the grain size is controlled to ASTM 7or finer. Recrystallizing to obtain a grain I size of ASTM or finer isdifficult to control industrially above a temperature of about 800 F.The intermediate gauge sheet is then cold rolled to effect a reductionin thickness between about 50 and percent of the intermediate gaugethickness. Cold rolling is usually conducted at less than 250 F.Preferably, the hot rolling is conducted in such a manner and at such atemperature to effect recrystallization during the hot rolling operationto avoid a subsequent recrystallization annealing step. Normally the hotrolling temperature is maintained above 600 F preferably over 700 F.With multistand mils it is usually required to slow down the mill speedto provide sufficient time to allow for recrystallization.

It has also been found that the recrystallization anneal can be usedbetween the early cold rolling passes. However, at least half of thereduction during cold rolling should occur after the recrystallizationanneal to avoid orange peel. Orange peel is a surface roughening in theform of a grain pattern where the surface of the sheet has a large grainstructure and is stretched beyond its elastic limit.

As used herein the expression intermediate anneal includes substantiallycomplete recrystallization during hot rolling, before cold rolling orduring the initial phase of the cold rolling operations subject tolimitations set forth above.

It is believed that the ripple surface, which forms when the sheetmaterial is stretched during fabrication, is due to the presence ofstrong transgranular shear bands which are generated during the hotrolling and cold rolling steps normally employed in producing automotivetrim sheet material. The microscopic shear bands occur transverse to therolling direction at an angle between about 3045 from the surface of thesheet when viewing a longitudinal cross-section. It has been found thatthe shear bands must extend to or near the surface of the sheet beforethe rippled appearance develops appreciably during the stretchfabrication.

In accordance with the present invention it has been found that theshear bands which are initially developed during hot rolling can beeliminated or minimized by efiecting a substantially completerecrystallization during an intermediate anneal. Preferably at leasthalf of the reduction during cold rolling must occur afterrecrystallization, otherwise theresultant sheet products exhibit severeorange peel and substantially reduced strength which severely limits theusefulness of the sheet material for automotive applications. Tominimize the effect of orange peel the recrystallization should becontrolled in such a manner so as to obtain a grain size of ASTM 7 orfiner. The relatively high strength is necessary in these applicationsto prevent any loss in dent resistance which is a primary requirementfor automotive applications.

In the process described above, the alloy is normally hot rolled to agauge thickness of between about 0.08 and 0.20 in. and cold rolled to agauge thickness between about 0.02 and 0.10 in. The described thermaland rolling practices of the present invention provide for a sheetmaterial having a tensile strength between about 29,000 and 53,000 psi,an elongation between about I and 5 percent in 2 in. A microscopicexamination of the resultant product shows that it is characterized bythe absence of transgranular shear bands and further characterized bysignificant amounts of cube texture components which are determined byX-ray analysis. Preferably the cube texture components amount to between5 and 30 percent of the total texture. The rest of the structure isprimarily the normal rolling texture.

Aluminum sheet material containing magnesium greater than 3.8 percent,which has been cold rolled large amounts, e.g. 70-95 percent, isextremely hard and brittle. When this sheet is subject to fabrication inwhich the primary mode of deformation isby stretching, the sheet has atendency to crack or break, and, moreover, this tendency is noteliminated by heating the material to an elevated temperature such asbetween 400-550 F. Further, the resultant sheet material containing morethan 3.8 percent magnesium and the normal impurity levels, when brightdipped and anodized in a percent sulfuric acid electrolyte, does nothave the surface characteristics required for automotive trimapplications in that the specular reflectivity and image clarity isseverely reduced. Moreover, there is some evidence that sheet materialcontaining more than 3.8 percent magnesium has the tendency to formshear bands when subjected to large amounts of cold rolling, forexample, 80-95 percent. Sheet material containing less than 0.5 percentmagnesium does not have the required strength and dent resistance forautomotive applications.

The following is an example of the embodiment of the present inventionin comparison with the prior methods of producing auto trim material.The example is intended to be illustrative and it has been selected as atypical example to demonstrate the invention rather than to limit it.

A 5252 aluminum alloy ingot having a composition set forth below Ti Al0.009 Balance.

Si Fe Cu Mn Mg Zn was homogenized, scalped and preheated in a normalmanner to slightly above hot rolling temperatures. The ingot was thenpassed through a breakdown mill to reduce the thickness of the ingot toabout 1 inch. The 1 inch plate was hot rolled to a thickness of 0.102inch with a mill entry temperature of 875 F. and a mill exit temperatureof 540 F. Half of the hot rolled sheet was subjected to arecrystallization anneal at a temperature of 650 F. for about 2 hoursand then separate samples of the sheet were cold rolled to effect athickness reduction of 70, 80 and 90 percent respectively. The remaininghalf of the hot rolled sheet was divided into separate samples and theseparate samples were cold rolled to effect a reduction in thickness of70, 80 and 90 percent respectively. The samples ture of approximately450 that had been annealed to effect recrystallization exhibited notransgranular shear bands upon microscopic examination, and, when thesamples were subsequently stretch formed at a temperature ofapproximately 450 F., exhibited no rippled formation on the surfacethereof. The samples having no intermediate anneal after hot rolling,exhibited rather severe shear bands and severely rippled when stretchformed at a tempera- F. The annealed samples subjected to theintermediate anneal showed a slight diminution of reflectivity andstrength when compared to the unannealed samples, but the differenceswhere well within acceptable commercial limits. The tensile propertiesof both the annealed and unannealed samples are set forth in thefollowing table:

[Properties in the rolling direction] Ks. in

As indicated in the above example, the product of the present inventiondoes not form ripples when subjected to stretching during fabricatingprocedures and, furthermore, has the strength and reflectivityrequirements for automotive trim applications. Moreover, it should benoted that the product of the the present invention has no tendency togive an orange peel effect when subjected to a fabrication procedure inwhich the primary mode of deformation is stretching.

What is claimed is:

1. An aluminum-magnesium sheet suitable for trim stock which has beencold rolled between about 50 and percent characterized by improvedreflectivity and surface finish, by a tensile strength between about29,000 and 53,000 psi, an elongation between about i and 5 percent andby the absence of microscopic transgranular shear bands transverse tothe rolling direction extending proximate to the surface of said sheetat an angle between about 30 and 45 to said surface, said sheet having acomposition consisting essentially of 0.5 to 3.8 percent magnesium andthe balance aluminum, impurities, and incidental elements, saidimpurities and incidental elements not exceeding the limits of up to0.10 percent silicon, up to 0.15 percent iron, up to 0.10 percentcopper, up to 0.30 percent manganese, and other elements up to 0.05percent each up to 0.15 percent total.

2. The aluminum-magnesium sheet of claim 1 wherein said sheet has acomposition containing 2.0 to 3.6 percent magnesium.

3. The aluminum-magnesium sheet of claim 1 further characterized by amicro structure containing between about 5 and 30 percent cube texturecomponent.

2. The aluminum-magnesium sheet of claim 1 wherein said sheet has acomposition containing 2.0 to 3.6 percent magnesium.
 3. Thealuminum-magnesium sheet of claim 1 further characterized by a microstructure containing between about 5 and 30 percent cube texturecomponent.